1. Field of the Invention
The present invention relates to an electroacoustic transducer for converting electric signals to acoustic signals, and, more particularly, to an electroacoustic transducer, using an improved material and structure for lead terminals, which suppresses the precipitation of the component that hinders the solder wetting property (good solder wetting property means that solder is well adhered to the lead terminals), which ensures a thinner lead frame, which accelerates the local heating of lands at the time of soldering coil terminals to the associated lands to thereby make it unnecessary to increase the heating temperature or heating time, thus preventing resin parts in the vicinity of the lands from being thermally damaged, and which prevents the soldered portion from being remelted by heat at the time the electroacoustic transducer is mounted on the mount board of an arbitrary electronic device.
2. Description of the Related Art
Referring to FIGS. 26 through 33, the structure of a conventional electroacoustic transducer will be explained according to the fabrication steps. To begin with, the structure of a lead frame 201 shown in FIG. 26 will be described. The lead frame 201 has a plurality of lead frame elements 201a (each lead frame element is encircled by an imaginary line in FIG. 27) coupled side by side to ensure the fabrication of a plurality of electroacoustic transducers, and FIG. 26 shows one of the lead frame elements 201a. Each lead frame element 201a has a pair of frame guide rails 203 and four guide holes 204. Formed between the pair of frame guide rails 203 is space 205 in which projecting pieces 207, 209, 211 and 213 are protrusively provided. Those projecting pieces 207, 209, 211 and 213 have distal ends 207a, 209a, 211a and 213a which serve as lead terminals of a completed electroacoustic transducer. That is, the projecting pieces 207, 209, 211 and 213, when cut along cut lines A in FIG. 26, become the lead terminals 207a, 209a, 211a and 213a.
The aforementioned lead frame 201 has a structure as shown in FIG. 27 in which a plurality of lead frame elements 201a (four elements in this case) shown in FIG. 26 are coupled side by side. A base member 215 is formed on this lead frame 201. Specifically, as shown in FIGS. 27 and 28, the lead frame 201 is placed in a mold, and a yoke (base) 217 and the lead terminals 207a, 209a, 211a and 213a are formed by inserting (at this point of time, cutting along the cut lines A in FIG. 26 has not been carried out yet). At this time, the surface of the yoke 217 and the lands (soldering portions) of the lead terminals 207a, 209a, 211a and 213a are exposed. Note that a pole piece (core) 219 should have been formed previously in the center of the yoke 217.
Then, various parts should be assembled to the structure shown in FIGS. 27 and 28. First, a coil 221 is wound around the pole piece 219 of each lead frame element 201a, as shown in FIG. 29. Then, a support ring 223 is placed inside the base member 215, with a magnet 225 put in the ring 223. Then, a diaphragm 230, which has a magnetic piece 227 attached as an added mass to the center of an elastic plate 229, is set on the support ring 223. Next, both coil terminals 221a and 221b of the wound coil 221 are led on the lands of the lead terminals 207a and 209a and are soldered there and then cut away. The above-described work should be performed for all the lead frame elements 201a, thus yielding the state shown in FIG. 30.
As shown in FIG. 31, a case 231 should separately be formed of synthetic resin, and should be placed over the base member 215 of each lead frame element 201a. Next, the case 231 is securely welded to the base member 215 by ultrasonic welding as shown in FIG. 32. Through the work done up to this point, four electroacoustic transducers are fabricated on the lead frame 201. Finally, the projecting pieces 207, 209, 211 and 213 of each lead frame element 201a are cut along the cut lines A shown in FIG. 26 to separate the individual electroacoustic transducers from one another. Then, the lead terminals 207a, 209a, 211a and 213a of each electroacoustic transducer are subjected to a forming process, providing a completed electroacoustic transducer as shown in FIG. 33.
Each completed electroacoustic transducer is then mounted as informing means on an arbitrary electronic device such as a portable telephone or a pager. The electric connection on such an electronic device is generally made by reflow soldering.
The above-described conventional structure has the following shortcomings.
In general, an alloy such as brass or phosphor bronze is used for the above-described lead frame 201. Those copper alloys are characterized by their relatively low cost and high thermal conductivity. Those copper alloys contain a component like Zn or Be, which deteriorates the solder wetting property and which is precipitated on a soldered plated layer as time goes, thus hindering the solder wetting property. Conventionally, therefore, double plated layers of Ni and Cu are provided to suppress the precipitation of the component which hinders the solder wetting property.
The conventional lead frame 201, as has already been discussed above, is formed of an alloy of brass, phosphor bronze or the like which is relatively soft. Therefore, the required mechanical strength cannot be obtained unless the lead frame 201 is formed thicker to a certain degree.
The very fact that the material for the lead frame 201 has high thermal conductivity raises another problem. Specifically, in soldering both coil terminals 221a and 221b of the coil 221 to the lands of the lead terminals 207a and 209a, heat escapes from the lead terminals 207a and 209a toward the frame guide rails 203 of the lead frame 201. This impairs the local heating on the lands. It is therefore difficult to execute the desired soldering at the normal heating temperature, so that the heating temperature or the heating time is increased. When the heating temperature or the heating time is increased, however, resin parts in the vicinity of the lands may be damaged by the heat.
In mounting a completed electroacoustic transducer on the mount board of an arbitrary electronic device like a portable telephone or a pager, the reliability of the soldered portions of the coil decreases. As discussed above, the lead frame 201 has high thermal conductivity and is thick, so that heat is easily transferred to the inner portions of the lead terminals from the outer portions thereof at the time of soldering to the mounting board. Accordingly, solder on the previously-soldered portions of the coil terminals may be remelted, causing the coil terminal 221a and 221b to come off. In this respect, the melting temperature of solder should be set high enough that solder is not remelted by the heating temperature at the time an electroacoustic transducer is mounted on the mounting board.